1. Field of the Invention
This invention relates to a novel copolymer consisting of a cumarin derivative and N-vinylcarbazole, a process for producing the same, and an electroluminescent (hereinafter abbreviated to EL) element using the copolymer.
2. Description of the Related Art
Noticing the high fluorescent efficiency of organic compounds, research on elements utilizing the EL properties of organic compounds has been carried out for a long time. For example, W. Helfrish and Dresmer Williams obtained a blue color fluorescence using anthracene crystals (Journal of Chemical Physics, Vol. 44, p. 2902, 1966, and J. Chem. Phys., 44, 2902 (1966)). Further, C. W. Tang and S. A. Vanslyke obtained a green color fluorescence using an oxine complex (Applied Physics, Lett., 51 (12), 21 (1987)).
However, these have not been provided with sufficient conditions for practical utility. For example, there have been raised such problems that, in the former case, the driving voltage is high, and in the latter case, as the luminescent layer has no positive hole-injection capability, a laminate structure containing a positive hole-injecting layer is required, and further, it is difficult to obtain a luminescence having a wavelength shorter than the luminescent wavelength of 8-hydroxy-quinoline aluminum complex having a capability of electron-injecting capability.
On the other hand, Japanese patent application laid-open No. Hei 4-212286 discloses an organic EL element having a mixture of a compound having a positive hole-injecting capability, a fluorescent substance and a compound having an electron-injecting capability. However, this element, too, has been unsatisfactory in the aspect of luminescent efficiency. Further, Japanese patent application laid-open No. Hei 4-359989 discloses a cumarin polymer. However, the cumarin homopolymer has a drawback that the concentration extinction is notable. Thus, the luminescent efficiency is inferior.
In order to obtain a high luminescent efficiency in the organic EL element, it is necessary to efficiently take out the positive hole and electron from the electrode, efficiently transport them as far as the luminescent center and further, efficiently inject them into the luminescent center. As one of materials for transporting the positive hole and injecting it into the luminescent center as above (positive hole-transporting material), poly N-vinylcarbazole (hereinafter abbreviated to PVCz) has been known. An example wherein this PVCz was used as a positive hole-transporting layer, has been disclosed in Japanese patent application laid-open No. Hei 3-137186. However, a potential barrier between the layer and the luminescent layer has constituted a factor of lowering the efficiency. Further, the above Japanese patent application laid-open No. Hei 4-212286 discloses an example wherein a fluorescent pigment is mixed with PVCz. When the positive hole or electron moves from one molecule to another molecule, the shorter the distance between the respective molecules, the more efficiently the hole or electron moves. In other words, it is preferred that the positive hole-transporting material is close to the luminescent layer. Additionally, as compared with the case where the positive hole-transporting material is mixed with the luminescent material, a case where they are chemically bonded to each other is particularly preferred.